Known gas sensors are mostly based on chemical or optical sensing principles. For the most part, optical sensing principles employ an absorption process, as summarized in FIG. 4. The known infrared optical gas sensor 2 shown exemplarily in FIG. 4 is based on the principle whereby, in a gas mixture, such as air, in which the concentration of a gaseous component is to be measured, for instance, the CO2− concentration in the air, the gaseous component to be measured absorbs light at a characteristic wavelength of λgas in the infrared spectral region. If, in the case of light, which has propagated through gas 7, respectively the gas mixture along an absorption path 12, the light intensity at this characteristic wavelength λgas is compared to the light intensity at reference wavelength λref that is adjacent to the characteristic wavelength, that is not absorbed by the gaseous component to be measured, then the concentration of the gaseous component to be measured can be calculated on the basis of the known Lambert-Beer law of light absorption, as follows:I=I0*exp(−ε*c*L)  Equation 1I=the light intensity given an absorption of [W/m2]I0=the intensity without absorption [W/m2]ε=the molar absorption coefficient [m2/mol](gas-dependent, wavelength-dependent and temperature-dependent)c=the molar concentration [mol/m3]; andL=the length of the absorption path 12 [m].
To calculate the molar concentration c of the gas to be detected, Equation 1 requires knowing the known molar absorption coefficient ε, a measured value for the light intensity I with absorption (i.e., at the characteristic wavelength λgas), a measured value for the light intensity I0 without absorption (i.e., at the reference wavelength λref), and a measured value for the length L of the absorption path 12. The longer the absorption path 12 is, the more light that is absorbed, and the lower the molar gas concentration c that can be measured.
The basic design of an infrared optical gas sensor 2 shown exemplarily in FIG. 4 for measuring the concentration of CO2 in air, for example, includes a light source 10 that is broadband emitting in the infrared spectral region and whose emission spectrum includes the wavelength λgas that is characteristic of the gas CO2 to be measured, and a wavelength λref adjacent thereto at which the gas mixture (the air) does not absorb, and, in addition, a light detector 80 having at least two measuring channels for measuring the light intensity at the wavelength λgas that is characteristic of the gas component to be measured (also referred to herein as measurement wavelength), and at the reference wavelength λref. The light detector 80 includes a filter device 83 for allowing light of measurement wavelength λgas to pass through, and a measuring light detector 81 disposed downstream therefrom, a filter device 84 for allowing light of reference wavelength λgas to pass through, and a reference light detector 82 disposed downstream therefrom.
The purpose of the first measuring channel 88 of the light detector 80 is to measure the light intensity I remaining at measurement wavelength λgas following propagation through the absorption path 12. The purpose of the second measuring channel 89 is to measure the light intensity I0 that has passed through at wavelength λref, where the gas mixture does not exhibit any absorption. The absorption spectrum of the gas mixture (air) is shown exemplarily in the lower portion of FIG. 4. One can discern that the gas components H2O, CH4, CO2 and CO contained in the gas mixture exhibit a pronounced light absorption (relative absorption) at different wavelengths that are characteristic of the particular gas components. The upper horizontal axis of the absorption spectrum illustrated in the lower portion in FIG. 4 indicates the wavelength λ, expressed in micrometers [μm], of the infrared light; and the lower horizontal axis indicates the corresponding wave number, which is defined as the reciprocal value 1/λ of the wavelength, measured in [105 cm−1].
Thus, infrared optical gas sensors are known where the measurement path, respectively the absorption path is configured within the gas sensor, in particular within a housing of the gas sensor. To achieve a longest possible absorption path between the infrared light source and the light detector, the gas sensor includes at least one reflector device allowing it to realize a plurality of absorption path sections within the gas sensor, as in the case of the photometric gas sensor described in the German Patent Application No. DE 10 2004 044 145.
It is a characteristic of the known optical infrared gas sensors that, in spite of a sophisticated reflector device design, the entire absorption path is limited by the dimensions of the gas sensor. Thin, portable devices, such as cell phones, handheld measuring devices or laser distance measurement devices, in particular, have no available space for realizing such an absorption path lengthened by a reflector device therein.